Originally used to describe the pallor which developed amongst inveterate D&D players, it widened to include denizens of any stripe who resided in the basement of their parents. Most recently it is used to describe the complexion of any video game addict.

1978: His dungeon tan is proof enough that he is an indestructible wizard.
1998: "Yo, ghost boy ! Nice dungeon tan. I see that you gave up POGs for Magic Cards !"
2008: PlayStation and Xbox gave him the dungeon tan. But at least Wii gives him some muscle tone.